With advances in technology, there is a continuous demand to increase data transmission rates and volume of data transmission. Traditional communication lines, such as cable containing copper wire, have been used to cater to this continuously-growing demand. However, traditional communication lines are subject to many disadvantages. Traditional communication lines are susceptible to interference during the transmission of data. Examples of such interference include, but are not limited to, electromagnetic interference and inter-symbol interference. Traditional communication lines are also characterized by a limited bandwidth, thereby limiting the rate at which data are transmitted via traditional communication lines.
Fiber-optic telecommunication systems have been introduced to the telecommunication industry to overcome the shortcomings of traditional communication lines. Fiber-optic telecommunication systems are characterized by immunity to electromagnetic interference. Specifically, fiber-optic telecommunication uses optical fibers for the transmission of information. Since optical fibers transmit light, rather than electrons, optical fibers do not radiate electro-magnetic energy. In addition, fiber-optic telecommunication systems have high bandwidth for the transmission of information. As is known by those of ordinary skill in the art, high bandwidth permits higher signal transmission rates.
A basic fiber-optic network contains a transmitter, an optical fiber fiber-optic, and a receiver. The transmitter contains a light-emitting diode (LED), laser diode (LD), an edge-emitting laser, a vertical cavity surface-emitting laser (VCSEL), or any other device that converts an electrical signal into an optical signal for transmission via the optical fiber. The receiver typically contains a photodiode that converts the light received from the optical fiber back into an electrical signal, and an amplifier for amplifying the electrical signal to make detection of the electrical signal easier.
It is generally desirable for lasers that are used in fiber optic telecommunication systems to produce light at a single, relatively long wavelength, in the range from approximately 1.2-1.6 micrometers (μm). Light in the wavelength range from approximately 1.2-1.6 μm can be transmitted by an optical fiber over long distances with relatively little loss of signal intensity. However, light emission in the range of 1.2-1.6 μm requires the semiconductor laser to be fabricated from materials having corresponding bandgap energy. Photons in semiconductors are further discussed in B. Saleh et al., Fundamentals of Photonics, pp. 543-643 (1991), which is incorporated herein by reference.
A semiconductor laser capable of operating in the 1.2-1.6 μm wavelength region may be made using an active region of indium gallium arsenide phosphide (InGaAsP) grown on an InP wafer. However, InGaAsP has a small conduction band offset relative to InP, leading to performance degradation at high operating temperatures (i.e., greater than approximately eighty degrees Celsius). Therefore, semiconductor lasers incorporating InGaAsP require cooling systems, thereby increasing laser cost. In addition, it is difficult to grow high-quality distributed Bragg reflectors (DBRs) of a vertical cavity surface-emitting laser (VCSEL) on an InP wafer.
Indium gallium arsenide nitride (InGaAsN) is an attractive material for the active region of long wavelength edge-emitting lasers and VCSELs since InGaAsN can be grown on a gallium arsenide (GaAs) wafer. InGaAsN and GaAs have a high conduction band offset. Moreover, such devices can use GaAs/aluminum arsenide (AlAs) DBRs, which are more reflective than DBRs made of InP-compatible materials. However, using InGaAsN in a semiconductor laser gives rise to significant manufacturing challenges.
InGaAsN edge-emitting lasers and VCSELs capable of continuous wave operation can be grown by metal organic chemical vapor deposition (MOCVD), which is generally suitable for mass production. Dimethylhydrazine ((CH3)2NNH2, also denoted as DMHy) has been used as a nitrogen precursor in InGaAsN MOCVD growth. This nitrogen precursor, DMHy, has the advantages of a relatively low decomposition temperature and therefore is suitable for growing InGaAsN by metal organic chemical vapor deposition (MOCVD) at a typically low growth temperature (500-600 degrees Celsius). In addition, dimethylhydrazine is a liquid and has reasonably high vapor pressure at room temperature.
There are, however, disadvantages with using dimethylhydrazine as a nitrogen precursor. To obtain a laser capable of emission in the 1.3 μm range, for instance, InGaAsN having a composition of In0.38Ga0.62As0.99N0.01 is desired. However, due to extremely low N incorporation in InGaAsN using MOCVD with dimethylhydrazine as the N precursor, a dimethylhydrazine/total group-V precursor ratio of more than 95% is supplied to the MOCVD reactor to obtain an N/total group-V atoms ratio of 1% in solid composition. This results in a large consumption of dimethylhydrazine. In addition, the following InGaAsN quality issues result from use of dimethylhydrazine as a nitrogen precursor.
To enhance N incorporation in InGaAsN and to achieve a very high dimethylhydrazine/total group-V precursor ratio (e.g., more than 95%), a very low As/group III ratio (e.g., approximately 5) is required. However, a low As/group III ratio typically causes poor material quality in GaAs-based semiconductors. In an example characterized by a very high dimethylhydrazine/total group-V precursor ratio and a very low As/group III ratio, the ratio of the precursors for group III, As, and N is 1:5:100. With this ratio, the sticking coefficient of N is estimated to be approximately 0.0001.
Another negative aspect of using dimethylhydrazine as a nitrogen precursor is that the purity of dimethylhydrazine is currently not well-controlled, especially residual water level. Water contamination in dimethylhydrazine causes the grown material to have poor quality. As mentioned above, a laser operating with a wavelength of approximately 1.3 μm requires a large dimethylhydrazine/total group-V precursor ratio for InGaAsN growth. The large ratio requires a large amount of dimethylhydrazine to be supplied to the MOCVD reactor, resulting in opportunities for the incorporation of impurities into the InGaAsN.
In addition, InGaAsN having approximately 40% In is desired make an active region for a laser that is capable of light emission at 1.55 μm. In such a laser, approximately 3% N in the active region is also desirable. Unfortunately, such an In mole fraction tends to dramatically reduce N incorporation in InGaAsN. Therefore, it is difficult to make a laser incorporating InGaAsN for generating light at 1.55 μm. As a result, it is desirable to find a suitable way to enhance nitrogen incorporation in InGaAsN for semiconductor devices, such as semiconductor lasers and light emitting diodes (LEDs).